1. Field of the Invention
The present invention relates to a motorcycle having a main frame extending rearward from a head pipe, a cross member connecting between a first frame branch and a second frame branch of the main frame, and left right seat rails extending rearward from the first and second frame branches.
2. Description of Related Art
An off-road motorcycle has an engine mounted in a cradle-type body frame, and a seat mounted in the body frame at a position above the engine.
An example of such a body frame is described in JP-A 2002-362464 and includes left and right main frames extending obliquely downward toward the rear from a head pipe, a cross member connecting between upper rear ends of the main frames, and left and right seat rails extending obliquely upward toward the rear from the main frames. An upper end of a rear cushion is connected to a bottom of the cross member, and front ends of the seat rails are connected to a top of the cross member.
In JP-A 2002-362464, an intake passage connected with an engine passes below the cross member and extends rearward and generally linearly to be connected to an air cleaner.
When an intake passage extends rearward from an engine linearly, as in this conventional motorcycle, an axis of the intake passage intersects with a cylinder axis generally perpendicularly. This structure is disadvantageous in that intake resistance is increased, thereby decreasing charging efficiency.
To decrease intake resistance, the angle formed between the axis of the intake passage and the cylinder axis may be minimized to form what is called a down-draft intake system. Such a system can be effectively built by extending the intake passage across and beyond the cross member to connect the intake passage to the engine obliquely from above.
However, to build such a system in which the intake passage extends across and beyond the cross member, the cross member must be lowered to avoid interference with a seat, and the positions at which seat rails are mounted must therefore also be lowered. This increases the distance between the seat rails and the seat, possibly causing the seat rails to fail to fully support a load imposed on the seat by a rider.